The 35 sect members, including 4 children, went underground in the Penza Region last November with the aim of saving "themselves during the time of the apocalypse," which they say will come this May.
They have threatened to set fire to themselves if any attempt is made to force them to come to the surface.
"We are ready to do anything that is required to help these people, but we are not going to provoke them into any kind of life-threatening action," the source said.
He also added that the authorities had contingency plans in place to save the sect members if thaw waters threatened to flood the cave.
The sect's leader, Pyotr Kuznetsov, 43, is currently undergoing psychiatric evaluation in an asylum in Penza, about 600 km (370 miles) southeast of Moscow. He was charged on Monday with inciting religious and racial hatred.
